Demand for secondary batteries as an energy source has been significantly increased due to the miniaturization and weight reduction in various electronic devices and the expansion of the electric vehicle market. Among these secondary batteries, lithium secondary batteries having high energy density, high voltage, long cycle life, and low self-discharging rate have been commercialized and widely used. Also, in line with growing concerns about environmental issues, a significant amount of research into electric vehicles and hybrid electric vehicles which may replace vehicles using fossil fuels such as gasoline vehicles and diesel vehicles, one of major causes of air pollution, has been conducted. Recently, research into the use of lithium secondary batteries having high energy density and discharge voltage as power sources of the electric vehicles and hybrid electric vehicles has been actively conducted.
In general, a lithium secondary battery is composed of a structure in which an electrode assembly, which is composed of a positive electrode including a lithium transition metal oxide as an electrode active material, negative electrode including a carbon-based active material, and a porous separator, is impregnated with a lithium electrolyte. In this case, the positive electrode is prepared by coating an aluminum foil with a positive electrode material including the lithium transition metal oxide, and the negative electrode is prepared by coating a copper foil with a negative electrode material including the carbon-based active material.
A conductive agent is added to the positive electrode material and the negative electrode material in order to improve electrical conductivity of the active material. In particular, since the lithium transition metal oxide used as the positive electrode active material has inherently low electrical conductivity, the conductive agent is essentially added to the positive electrode material. Among conductive agents, a chain-type conductive agent is generally used to increase the conductivity of the positive electrode material, wherein the chain-type conductive agent may not increase loading density during a compression process for reducing the thickness of the positive electrode material.
In a high energy density secondary battery, amounts of conductive agent and binder added are limited to increase the amount of a positive electrode active material included in a positive electrode material. In a case in which a large amount of graphitized carbon having insufficient electrical conductive is used as the limited amount of conductive agent, an electrical conductivity deviation occurs in the coated positive electrode material, and this may cause non-uniform performance of cells. The non-uniform performance of the batteries may cause a lot of limitations by causing an abnormal operation of the battery in a medium and large-sized device in which the plurality of batteries is used.
Thus, there is a need to develop a positive electrode material having a new configuration which may improve the above limitations.